Film and television image generation systems give rise to the appearance to the viewer of continuously moving visual images. Actually, the appearance of continuous motion results from visual and mental integration by the viewer of rapidly advancing sequences of still frame images.
Conventionally, in countries having a 60 Hz primary power distribution frequency, motion picture films are generated and are projected at one frame rate, such as 24 frames per second, while television images are generated and displayed at another frame rate, such as 30 frames per second (60 dual-interlaced fields per second, or more precisely 59.94 Hz for the NTSC color standard).
Line scan doubling techniques have been proposed to overcome some of the drawbacks resulting from the early adoption of the 262.5 line per field scan standard for television. When the number of scan lines per field is doubled, and the result is presented in progressive scan per field format, improved visual appearance, particularly with large screen display formats, is realized.
The combination of conventional three to two pulldown ratio on film-to-video transfers and concomitant scan line doubling at the picture display gives rise to an unpleasant "shimmering" or "ragged edge" quality with moving picture images (transitions) in the resulting picture display content.
FIG. 1 illustrates a conventional prior art approach to film-to video transfer with line doubling. Graph A marks a time scale wherein each division represents one sixtieth of a second. Graph B denotes the time intervals of six film frames, for example, frame A, frame B, frame C, frame D, frame E, and frame F. At a 24 frame per second rate, each frame covers a 2.5/60th second still frame interval of action. As filmed, and as conventionally projected outside of television, each frame is projected for the same time interval. The projector light beam is nominally cut off while the film is advanced one frame, and the next frame is then projected during the uniform projection interval, and so forth.
Graph C of FIG. 1 illustrates the scan periods of odd and even television fields at the conventional 60 field per second rate. It is immediately apparent that a time discrepancy exists between the television field (graph C) and the film frames (graph B). Conventionally, this discrepancy has been resolved by projecting the first film frame A e.g. for three television fields, the second film frame B for two television fields, the third film frame C for three fields, the fourth film frame D for two fields, the fifth film frame E for three fields, the sixth film frame F for two fields, etc. This method yields the three to two film-to-video transfer with resultant conventional interlaced video.
Line doubling from e.g. 525 scan lines to 1050 scan lines has been proposed in the prior art in order to increase the apparent vertical dimension resolution of television picture displays and to overcome drawbacks otherwise associated particularly with large screen picture displays. FIG. 2A graphs a hypothetical, exemplary seven line, 2/1 interlace video system (wherein a low number of lines (seven) is selected to aid clarity of understanding). Graph A shows an odd field comprising scan lines 1, 3, 5 and one half of 7, and an even field including one half of scan line 7, and scan lines 2, 4 and 6, in a repeating pattern. FIG. 2B illustrates a simplified structure 10 to achieve line doubling of the graph A scan pattern. This exemplary prior art structure 10 omits motion detection and modification structure also to aid clarity in understanding. Incoming video at the conventional scan rate (graph A of FIG. 2A) branches at an input 12 into two paths, including a path leading through a 2/1 time compressor 14 to a summing junction 16 and an output 18. The time compressor 14 compacts the original scan line information into one half of the original time devoted to the scan line.
The other path leads through a one field delay 20 and a 2/1 time compressor 22 to the summing junction 16 and output 18. The time compressor 14 provides the FIG. 2A, graph B, time compressed scan lines 1, 3, 5, 7, 2, 4, 6, 8, etc. The one field delay 20 provides a one field delay offset to the incoming video (FIG. 2A, graph C) and the second 2/1 time compressor 22 time compresses the graph C delayed scan lines to provide the scan lines graphed as D in FIG. 2A. The summing junction 16 combines the graphs B and D time compressed line scans into progressive scan line doubled frames at the original field rate, wherein each frame contains twice the number of scan lines as the original field, as graphed in graph E of FIG. 2A.
When line doubling techniques are applied to conventional 3/2 pull down film-to-video transfers, temporal discrepancies become manifested between even and odd scan lines in the presence of motion. In order to achieve conventional line doubling, as explained in conjunction with FIG. 2 above, the film-to-video transfer is delayed by one field period (1/60th of a second) as shown in graph E of FIG. 1 and is combined as shown in graph F thereof in the field domain. In FIG. 1 graph F, the temporal discrepancies become apparent, since the resultant odd and even fields do not line up.
During a first time interval U, which nominally is 1/30th of a second and represents film frame B, the odd line scans for frame B are beginning before the even line scan for frame B. During the next, longer 1/20th of a second time interval V for film frame C, the odd scan lines are likewise occurring before the even lines. During the third, 1/30th second interval W for film frame D and the fourth, 1/20th second interval X for film frame E, the situation reverses, and the even scan lines are occurring before the odd scan lines. FIG. 3 graphs the repetitively inverting scan line displacements in the line doubled display. The repetitive inversions of lag and lead between pairs of film frame to video transfers results in the undesirable "shimmering" appearance in the display whenever there is image "motion" (i.e. spatial displacement between like subject matter in successive film frames). While dynamic motion detection methods and circuits have been proposed to correct this defect, e.g. by averaging scanning lines when motion is detected in the picture content, motion detection often fails; and, when it does work, line averaging results in a reduction of vertical resolution.
A hitherto unsolved need has arisen to achieve line doubled fields which do not contain these unwanted artifacts otherwise resulting from 3/2 film-to-video, line doubled transfers in the presence of motion.